The present invention relates to a method of manufacturing panels of water-laid fibers, preferably wood pulp fibers, which have a surface that is hard, water resistant and flame resistant.
For considerable time now fiber panels have been manufactured by interfelting a water slurry of fibers on a moving collection screen, consolidating and dewatering to form a wet mat, and drying the formed board, with or without optional wet or dry end coating, texturing and the like features. Improving certain of the surface properties by co-manufacturing a denser layer of the core composition is also known in the art. Methods for doing this are described in U.S. Pat. Nos. 3,779,860 and 4,153,503.
Heretofore wood fiber insulation board products produced by these means have been unable to obtain restricted flame spread classifications without expensive metal cladding or general fire retardive agent treatment. Further heretofore such boards have had very limited washability and water resistance due to wicking and retention of moisture into the fiber body.
It has now been discovered that the foregoing limitations may be overcome by the propitious application of a sodium silicate and calcium carbonate slurry.
It is known in general that soluble sodium silicate solutions may be employed as binders in fiber products; see for example U.S. Pat. No. 2,705,198. It is further known to react the silcate to water insoluble forms in certain of such products by adding a highly alkaline reactant such as hydrated lime and moderate heat or an ionic alkaline calcium compound with heat over 600.degree. F.; see for example U.S. Pat. Nos. 2,237,337 and 3,950,218. Philadelphia Quartz Company bulletin T-17-52 indicates resistance to rehydration of soluble silicates may be increased by slow baking to about 200.degree. F. followed by 300.degree.-400.degree. F. curing; higher temperatures may be used to unsolubilize the film; and sometimes permanency may be achieved by reaction with acid or solutions of calcium, magnesium or aluminum salts.
None of these investigations however have reported any evaluation of soluble silicate in combination with a calcium inert filler with short and moderate temperature drying.
Now it has been discovered that important advantages from both the processing viewpoints and the product viewpoints are gained when a still wet water-laid mat is coated with an aqueous slurry of certain soluble silicate solutions filled with particular proportions of calcium carbonate calculated to cause a limited penetration of the wet mat surface without segregation and then confined in medium heat for a short time.
It is an object of this invention to provide a novel method for preparation of a surface coated fiberboard of improved properties and characteristics.
It is another object to provide a wood insulation board having a surface with restricted flame spread characteristics, high modulus of rupture, and high washability approaching that of pre-finished wood hardboard.
It is yet another object of this invention to provide a process for the production of coated fiberboard wherein a soluble silicate coating is rendered insoluable without caustic reactive calcium compounds and lengthy baking or high temperature reactive conversion.